The best way I can describe Mezcal is that it is much bolder than Tequila in every way, stronger aromas, stronger flavors, etc. It helps to have an experienced palate, otherwise it may be awhile before you give it another chance. And since most brands still cook / smoke the Agave in earth pits, it helps if you like smoky flavors.
I personally like Mezcal, just as much or more than Tequila. I have approx. 40 listed here:
Mezcal Reviews - Tequila Brands, Ratings and Reviews - TEQUILA.net
Favored brands include Del Maguey, Scorpion, METL 2012, Sombra, and Jaral de Berrio
Check out Mezcal.com and some of the videos here.